Microphone for the measurement of transient pressures in a body of water



April 1961 J. F. KEITHLEY ETAL 2,979,691

MICROPHONE FOR THE MEASUREMENT OF TRANSIENT PRESSURES IN A BODY OF WATER Original Filed March 25. 1945 2| SHIM l2 LAMINATED MAGNETIC l4 MATERIAL tates MICROPHONE FOR THE MEASUREMENT OF %IEKATIE%IENT PRESSURES IN A BODY OF Joseph F. Keithley, 418 Rittenhouse St. NW., and Matthew L. Sands, 2107 Fort Davis St. SE., ,both of Washington, D.C.

Unite Original application Mar. 25, 1943, Ser. No. 480,576,

now Patent No. 2,715,717, dated Aug. 16, 1955. Dixisdfgfind this application Aug. 18, 1954, Ser. No. 9

1 Claim. (Cl. 340-12) (Grantedunder Title 35, U.S. Code (1952), sec. 266) lake, a landlocked sea or an arm of-the sea having a restricted entrance, which varies in period from a few minutes toseveral hours. It is sometimes desirable to atent studysuch phenomena which may occur over either short or long periods of time in places which are diflicult of access and under conditions which are not likely to be repeated to determine whether or not they act in concert or inopposition and the manner in which they modify eachotherr v In attempting to measure these phenomena, difficulties are causedby the fact that each of them has a frequency and an amplitude which varies from the frequency and amplitude of each of the other phenomena so that in designing an apparatus for the measurement thereof, it has been found diflicult to produce one which will simultane0usly respond to both low frequencies and high frequencies of varying amplitudes with equal accuracy and facility. Another ditficulty arises from the fact that in devices in which a microphone is used to measure transient pressure phenomena at various depths in a body of water, it is found'that the microphone does not have a linear response and a constant sensitivity at different depths due to the fact that the diaphragm of the microphone changes its flexibility with changes in pressure thereon. For instance, as the pressure increases on a microphone diaphragm the flexibility of the diaphragm decreases and, conversely, as the pressure on the diaphragm decreases the flexibility increases. Where it is desired to measure pressures throughout a range of depths, this leads to inaccuracies in the results as it is difficult to correlate the results produced at one depth with those produced .at another depth.

In making measurements of subsurface pressure phenomena, the variations therein are of a transient nature sothat it is impossible to analyze them into their various components on the spot. While a recorded trace may be made by means of a recording instrument and the trace subjected to future leisurely study, it is not in aformwhich may readily be analyzed to determine the component frequencies thereof and the magnitudes of the pressures at the respective frequencies.

One of the objects of the present invention is the provision of a new and improved apparatus for analyzing transient pressures in a body of water in which the fore Patented Apr. 11, 1961 V 2 going difiiculties are obviated and which is adapted to perform all the functions :of the devices heretofore proposed or now in general use.

Another object of this invention is to provide a novel microphone for measuring subsurface pressures in a body of water which microphone will have a substantially constant percentage change of inductance with respect to the measured pressure throughout a predetermined depth range in which measurements are to be made.

Another object of the invention is to provide anovel method of adjusting a microphone to be employed in measuring subsurface pressures in a body of water so that said microphone will have a substantially constant percentage change of inductance with respect to the measured pressure throughout a predetermined depth range in which measurements are to be made.

In general, in an apparatus of the character comprising the present invention, a novel watertight microphone, adjusted to give a substantially linear response and a constant, sensitivity for the particular range of depths at which measurements are to be made, is adapted to be submerged in a body of water, the output of the microphone preferably being connected to a network which converts the signal into a form which is readily recordable on a phonograph record.

More specifically, the present invention provides a system wherein a microphone adapted for underwater use is submerged below the surface of a body of water at depths through which desired measurements are to be made. i The microphone comprises a casing having therein a flexible diaphragm which carries an armature which cooperates with a U shaped core having a coil on each leg thereof. The microphone is provided with a novel feature in that the diaphragm and thecore are adjustable with respect to one another so that for a particular range of depths in which measurements are to be made, the diaphragm may be adjusted with respect to the core to produce, with change in pressure on the diaphragm, ,a linear response in the change of inductance of the windings on the core when these windings are supplied with alternating current.

That this result is obtained is evident from the following explanation. It will be noted, as pointed out above, that the flexibility of the diaphragm changes with. the change in pressure thereon. The rate of change of inductance of the windings on the core also changes as the armature attached to the diaphragm changes its .distance from the core with change in pressure. However, when the microphone is submerged and the flexibility of the diaphragm decreases with increase in pressure, the armature carried by the diaphragm is forced by the increased pressure to reducethe gap between it and the core and, therefore, to increase therate of change of inductance of the coils carried by the core. By proper adjustment of the initial gap between the armature and the core, it is possible for one of these factors to compensate for the change in the other factor substantially throughout a particular range of depths in which measurements are to be made.

The microphone of the present invention is particularly well suited for studying signals for all frequencies up to a desired value so as to determine the effects of sound,

turbulence, waves, tides and seiches at different he permeable magnetic material such as Permalloy. I p

' 'Atitheupper end of the casing 11, an -internal shoulnet 26 is provided. ,A disc 27 rests on said shoulder and isiarrang'ed to support a cylindrical body 28 of elastic 'material'such .as soft rubber; ,Adisc 29 similar to the disc39 rests upon the elastic material, the two discs being forced toward each other by means of bolts 31 In the single figure of the drawing there is'showni thereon a microphone unitdesignated. generally by: the

i numeral and illustrated in detail.

known as Permalloy inthe trade The U-shapedmember 12' is attached to a disc 13 preferably formed of a non-magnetic material, the disc and Ushaped member being fastened together by any well known means.

The

. be made without departing fromthe spirit and scope ofi the invention,iand it is intended, therefore, in the appended claims to cover all, such changes and :modi

disc 13 is 'attachedto a cylinder '14 by cap screws 15,

the cylinder 7 also, being formed of a non-magnetic material.

Surrounding each le'g orcore of the U-shaped mem ber 12 is a coil ,16,v the two coils being connected in series to a, cable 17; The casingll is permanently, at-

tached to an annular member 18 which supports the i cylinder 14and which is attached thereto by cap screws: 7 '19, 'Betweenthecylinder 1,4'ancl the disc13' is placed a shim 21 for a purpose: which will be describedhereinafter." The annular member 18 is provided with eye I 'bolts 22; to which lowering cables may be attached for I thepu'rpose of submerging the microphone in a bodyof water. 'Clatnpcd to the annular member 18is a dia- I phragm 23, the diaphragm'bein'g held in place by bolts" 24,thediaphragm; 23 being lapped totheannular mem- I ber: '18 i for the purpose of preventing leakage of water. into the interior of the, casing 11. The diaphragm 23. I carries a laminated Earmature 25 in alignment with the I pole faces 'of the-U-shaped member or core '12and equidistantly spaced therefrom and which'i's made of. a'highly and nuts 32. The discs 27 and 29 are provided with apertures 33 and 44 respectively through which pass the cable 17, the elastic material also being provided with an aperture in alignment with the apertures 33 and '34. This arrangement provides a watertight seal for the upper end of the casing 11 as tightening of the nuts 32 will force the elastic material between the discs 27 and 29 into close engagement with the walls of the casing 11 and with the cable 17. s

Referring to the single figure on the drawing, the arrangement shown therein is carried to a location on a body of water at which the desired measurements are to be made. The microphone unit 10 is submerged in the water to the desired depth by means of the eye bolts 22 and cables (not shown), the conducting cable 17 being unreeled a sufiicient amount to permit this. Prior to submergence of the microphone, a shim 21 of the proper thickness is placed within the microphone, as illustrated on the drawing, so as to space the armature 25 equidistantly from the pole faces 35 of the U-shaped member 12 a distance which will render linear the response of the microphone at the particular range of depths through which measurements are to be made as explained hcrcinabove.

Briefly stated in summary, the present invention contemplates the provision of a new and improved apparatus for analyzing transient pressures in a body of water which is suitable for use with means adapted to record accurately all of such pressures in a form which may easily be analyzed at leisure and which may be studied 4 minutely at all desired frequencies withoutthe. necessity of repeating the measurements. I I g While the inventionhasbeen described in detail with respect to a :certain preferredexample thereof which gives satisfactory results, it will be understood by those skilledin the art, after understanding the invention, that various features of i the "system disclosed and claimed herein; may advantageously be employed in types of sound or pressure recording systems other than that dis-f closedand that various modificationsand changes may fications.

The invention herein'described and claimed may be manufactured and used by or for the Government ofthe :United States of America for governmental purposes 7 without paymentof any royalties thereon or therefor; I j i What is claimed asnew and desired to be secured by Letters Patent of theUnited States isi v A microphone for use under waterfor measuring pres I "sure at various depths as an alternatingcurrent{is'applied thereto, in combination, an annular; member K having a.

plurality of eye bolts securedthereto' for supporting the member at a depth .of'submergencc within the'water, a

' circular diaphragm secured, to said member incoaxial and I watertight relation therewith and having a lobe at: a cen tral portion of the inncrface thereof and formed integrally therewith,'a laminated armature attached to said lobe and 1 .rnovable therewith, a laminated, 'U-shapedcore having j I fa pair of coplanar pole faces thereon, a pair of coils I t I carried by'SElid: U-shaped core-on the legs thereof, an a electric: jcable :having a pair of, conductors thereingcon-t nected to said coils for establishing an external electrical I i c'onnectionthereto, means including acasing carried bysaid annular member for establishing a watertight seal 7 with said cable, a "hollow rigid cylindrical member 'disposed within gsaid casing and secured to saidannular. i member, said cylindrical member enclosing but ;not in contact with said coils, a circular plate having'a'diarncter i equal to the outside diameter of saidlcylindrical member to which said U-shapcd core is secured at a medial portion thereof for supporting said U-shaped core such that thearmature is continuously aligned with said pole faces and in closely spaced equidistant relation therewith, said circular plate having a plurality of apertures therein, a plurality of screws disposed respectively within said apertures and threaded into said cylindrical member for clamping the plate to the cylindrical member, and an annular shim clamped between the adjacent clamping surfaces of said plate and cylinder for setting at will the initial spaced relation of the armature with respect to said pole faces without changing the aligned relation of the armature therewith in accordance with the thickness of said shim.

References Cited in the file of this patent UNITED STATES PATENTS 1,375,707 King Apr. 26, 1921 1,522,289 Cover Jan. 6, 1925 1,523,540 Hecht Jan. 20, 1925 1,571,454 Morrison Feb. 2, 1926 1,587,828 Gernsback June 8, 1926 1,677,945 Williams July 24, 1928 1,940,553 Lieber -s Dec. 19, 1933 2,346,429 Harrison Apr. 11, 1944 2,410,805 Black Nov. 12, 1946 2,505,519 Beranek Apr. 25, 1950 

